I take your point but sigil is an epub processor and an epub with a strange file type is still really an epub, inside....

Yes. And by that same logic, surely you can load the "book.original_epub" file onto all epub devices (or into all epub reading apps) and it will handle/read it just fine, right? Why not? It's still really an epub isn't it? Can you submit a "book.original_epub" file to online retailers who accept ePub submissions? Will online ePub validators work with it?

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is there not something inside a file that tells windows what it "really" is & could not sigil not look at that, rather than at the filetype shell.

I've thought of several responses here, but ultimately, I feel that "No" is the best one. Also keep in mind ... same code, three different platforms.

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& yes I think it should open zips if the zip is really an epub. I am surprised it does not already do zip.